Respiratory systems provide breathable gas, such as oxygen, anesthetic gas and/or air directly to a patient's mouth, nose or airway to assist or facilitate breathing by the patient. A ventilator may be used as part of the respiratory system to drive the breathable gas to the patient through an inspiratory limb hose or conduit of a breathing circuit. The breathing circuit may also include expiratory limb hose or conduit to carry expelled air and other gas(es) from the patient back to the ventilator or to atmosphere.
It is typically desired to warm and impart humidity to the breathable gas before it is provided to the patient. For that purpose, many respiratory systems include a humidification system having a heater unit and a disposable water chamber adapted to be heated by the heater unit. The heater unit supports a heater, such as a hot plate heater which may be comprised of one or more heating elements and a metal plate defining a hot plate. A wall of the chamber, such as the bottom surface thereof, is thermally conductive. The chamber is removably supported on the heater unit with the bottom surface in thermal contact with the hot plate of the heater unit to thus heat the water in the chamber. The chamber may be manually refillable, or there may be a water source to selectively fill the chamber as it empties. The breathable gas is coupled to the chamber and is passed through the chamber to be heated and humidified. Examples of heater units, chambers and vented water supplies are shown in U.S. Pat. Nos. 7,722,016, 6,988,497, and 5,943,473; and U.S. Patent Publication No. 2008/0054497.
The inspiratory limb carries the heated and humidified gas to the patient and the expiratory limb, if present, carries exhaled air and possibly other gases from the patient. The hoses or conduits of the inspiratory and expiratory limbs may each be provided with a heater, referred to as a heater circuit, to add heat to the gas passing through the limb and to avoid the incidence of potentially dangerous or hazardous rainout of condensation from the gas(es) passing through the limb. The heater circuit may be in the form of one or more elongated, and possibly coiled, heater wires running along the limb, such as through the interior of the limb. An example of a breathing circuit with heated limbs is shown in U.S. Pat. No. 6,078,730. The heater unit typically houses the necessary electrical and electronic components to regulate the temperature of the hot plate, as well as heating circuits of the inspiratory and/or expiratory limbs of the breathing circuit.
Regulating the temperature of those heaters can be essential to proper operation of the respiratory system. By way of example, a single point of failure in the control path for the heater, such as in the hardware or software of the electronic components in the heater unit, could lead to undesirable overheating of the hot plate or one of the heater circuits in the breathing circuit. Depending on the level of overheating, the consequences can be quite serious. While some level of protection is obtained with safety cut-offs, such as in the form of a fuse or thermal breaker or the like, additional levels of control are desirable to reduce the risk of overheating from a single point of failure.